1. Field of the Invention
This invention relates to a means and method for preventing build-up of involatile compounds and subsequent flow blockage in the source reagent vaporization zones of chemical vapor deposition reactors. The means and method of the invention protect such vaporization zones from accumulation of solid or liquid material that may result in clogging by modifying the thermal conductance and surface area of the internal parts of the vaporization zone.
2. Description of the Related Art
Recently many refractory materials have been identified as having unique materials properties. The recently discovered high temperature superconducting (HTSC) materials include YBa.sub.2 Cu.sub.3 O.sub.x, wherein x is from about 6 to 7.3, BiSrCaCuO, and TlBaCaCuO. Barium titanate, BaTiO.sub.3, and barium strontium titanate, Ba.sub.x Sr.sub.1-x TiO.sub.3, have been identified as ferroelectric and photonic materials with unique and potentially very useful properties. Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 is a photonic material whose index of refraction changes as a function of electric field and also as a function of the intensity of light upon it. Lead zirconate titanate, PbZr.sub.1-x Ti.sub.x O.sub.3, is a ferroelectric material whose properties are very interesting. The Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are materials that are useful for scintillation detecting and coating of optical fibers. Refractory oxides such as Ta.sub.2 O.sub.5 are seeing expanded use in the microelectronics industry; Ta.sub.2 O.sub.5 is envisioned as a thin-film capacitor material whose use may enable higher density memory devices to be fabricated.
Many of the potential applications for these materials require their use in thin film, coating, or layer form. The films or layers may also be advantageously epitaxially related to the substrate upon which they are formed. Applications in which the refractory materials may need to be deposited in film or layer form include integrated circuits, switches, radiation detectors, thin film capacitors, holographic storage media, and various other microelectronic devices.
Chemical vapor deposition (CVD) is a particularly attractive method for forming these layers because it is readily scaled up to production runs and because the electronic industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to new CVD processes. In general, the control of key variables such as stoichiometry and film thickness, and the coating of a wide variety of substrate geometries is possible with CVD. Forming the thin films by CVD will permit the integration of these materials into existing device production technologies. CVD also permits the formation of layers of the refractory materials that are epitaxially related to substrates having close crystal structures.
CVD requires that the element source reagents must be sufficiently volatile to permit gas phase transport into the deposition reactor. The element source reagent must decompose in the reactor to deposit only the desired element at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds are desired to be deposited, obtaining optimal properties requires close control of stoichiometry which can be achieved if the reagent can be delivered into the reactor in a controllable fashion. In addition, the reagents must not be so chemically stable that they do not react in the deposition chamber.
Thus a desirable CVD reagent is fairly reactive and volatile. Unfortunately, for many of the refractive materials described above, volatile reagents do not exist. Many potentially highly useful refractory materials have in common that one or more of their components are elements, such as the Group II metals barium, calcium, or strontium, or early transition metals zirconium or hafnium, for which no volatile compounds well-suited for CVD are known. In many cases, the source reagents are solids whose sublimation temperature may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, and it will be very difficult to control the stoichiometry of the deposited films.
When the film being deposited by CVD is a multicomponent substance rather than a pure element, such as barium titanate or the oxide superconductors, controlling the stoichiometry of the film is critical to obtaining the desired film properties. In such materials, which may form films with a wide range of stoichiometries, the controlled delivery of known proportions of the source reagents into the CVD reactor chamber is required.
In other cases, the CVD reagents are liquids, but their delivery into the CVD reactor in the vapor phase has proven problematic because of problems of premature decomposition or stoichiometry control. Examples include the deposition of tantalum oxide from the liquid source tantalum ethoxide and the deposition of titanium nitride from bis(dialkylamide)titanium reagents.
The problem of controlled delivery of CVD reagents into deposition reactors was addressed by the inventors in U.S. patent application Ser. No. 07/807,807, which is a continuation of U.S. patent application Ser. No. 07/549,389, "Method for Delivering an Involatile Reagent in Vapor Form to a CVD Reactor," and further elaborated in U.S. patent application Ser. No. 07/927,134, "Apparatus and Method for Delivery of Involatile Reagents," which hereby are incorporated herein by reference. As described and claimed in these patents, the delivery of reagents into the deposition chamber in vapor form is accomplished by providing the reagent in a liquid form, neat or solution, and flowing the reagent liquid onto a flash vaporization matrix structure which is heated to a temperature sufficient to flash vaporize the reagent source liquid. A carrier gas may optionally be flowed by the flash vaporization matrix structure to form a carrier gas mixture containing the flash vaporized reagent source liquid. These "liquid delivery systems" have addressed many of the problems of controlled delivery of CVD reagents.
While these liquid delivery systems present distinct advantages over conventional techniques, there is often some fraction of the precursor compound that decomposes into very low volatility compounds that remain at the vaporization zone. This problem is a important issue in CVD processes that use thermally unstable solid source precursors which display significant decomposition at conditions needed for sublimation. Such decomposition can occur in all reagent delivery systems that involve a vaporization step, not only in the vaporizer in a liquid delivery system as described above but also in more conventional reagent delivery systems that include bubblers and heated vessels operated without carrier gas.
Although well-behaved CVD precursors vaporized under "ideal" conditions will form no deposits or residue at the vaporization zone, deviations from this situation are common and can be divided into several categories:
1) Reactive impurities in either the precursor or in the carrier gas decompose at the vaporizer temperatures.
2) Spatial and temporal temperature variations occur in the vaporization zone, with temperatures in some regions being sufficient to bring about decomposition.
3) CVD precursors are employed which are thermally unstable at the sublimation temperature.
Optimization of the conditions used in the vaporizer of reagent delivery systems can minimize the fraction of the delivered precursor that decomposes (and remains) at the vaporization zone, but virtually all solid and liquid precursors undergo some decomposition when they are heated for conversion to the gas phase, although this fraction is negligibly small in "well-behaved" compounds. Use of precursors that tend to decompose near their vaporization temperature may be mandated by availability (i.e., the selected precursor possessed the best properties of all available choices) or by economics, in the case where precursor cost is strongly dependent on the complexity of the synthesis.
Additionally, CVD precursors often contain impurities, and presence of those impurities can cause undesirable thermally activated chemical reactions at the vaporization zone, also resulting in formation of involatile solids and liquids at that location. For example, a variety of CVD precursors (such as tantalum pentaethoxide) are water sensitive and hydrolyzation can occur at the heated vaporizer zone to form tantalum oxide particulates that may be incorporated into the growing tantalum oxide film with deleterious effects.
Despite the advantages of the liquid delivery approach (which include improved precision and accuracy for most liquid and solid CVD precursors and higher delivery rates), this issue is the only serious impediment to widespread use of the technique and, accordingly, it is an object of the present invention to provide a means and method for extending the maintenance and cleaning cycles of vaporizers in liquid delivery systems used to introduce a variety of precursors to CVD reactors.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.